In order to facilitate communication between two telecommunications stations in a telecommunications system, transmission or repeater devices are required between the telecommunications stations in order to allow sufficiently interference-free signal transmission in both directions of transmission. For this purpose, for each direction of transmission there can be provided a separate transmission line which in the case of multiplex operation is employed in multiple fashion for a plurality of simultaneously existing connections between two telecommunications stations. A four-wire operation of this kind is preferably used at higher telecommunications network levels. In lower telecommunications network levels, and in particular in the region of the subscriber connection lines, generally signal transmission is carried out in duplex operation via two-wire lines. In this case the transmission signals associated with the two directions of transmission can be isolated by means of a hybrid or four-wire circuit which terminates the two-wire line and is comprised of a bridge circuit which converts the two-wire line, for example, into the four-wire section or portion of a subscriber end station (and vice versa). In order to achieve a complete decoupling of the receiving arm of the four-wire line leading away from the four-wire circuit from the transmitting arm of the four-wire line which leads into the four-wire circuit, the bridge circuit must be balanced and for this purpose must contain an exact simulation of the input impedance of the two-wire line. However, in order to ensure a high transmission quality by means of the four-wire circuit, in addition to the decoupling between transmitting and receiving arms of the four-wire line--which is attained when the bridge circuit is balanced--freedom of reflection between the four-wire circuit and the two-wire line is also desirable. This imposes corresponding additional demands on the bridge circuit.
In this respect a four-wire circuit is known (from German No. AS 17 62 849, incorporated herein by reference), which is comprised of a bridge circuit serving as a connecting element between a two-wire line and a four-wire line. The bridge circuit comprises an incoming and an outgoing component which entirely decoupled from one another but which are are each connected to the two-wire line. One arm of the bridge circuit is formed by the surge impedance of the two-wire line, and the second arm, which adjoins the first arm, is formed by a complex impedance which simulates the surge impedance of the two-wire line. The third and fourth arms of the bridge circuit are each formed by a resistor. Bridge circuit diagonals contain those parts of the four-wire line which are to be decoupled, together with their input impedances for the outgoing and incoming components. In this known bridge circuit the outgoing part of the four-wire line, for example the earphone of a telephone subscriber end station, is preceded by an impedance converter which reduces the input impedance of the outgoing component of the four-wire line to a value which is very low in relation to the surge impedance of the subscriber line. The incoming component of the four-wire line, for example the microphone of the telephone subscriber end station, is preceded by an impedance converter which raises the inner impedance of the incoming component of the four-wire line to a value which is very high relative to the surge impedance of the subscriber line.
A bridge circuit of this kind requires not only a transmitting arm located in one diagonal arm of the (balanced) bridge circuit with a very high (in principle infinite) inner impedance value, but also requires a receiving arm which is located in the other diagonal arm of the bridge circuit with a very low input impedance value, which in principle approaches zero. Such a design is provided, not only to achieve decoupling between transmitting and receiving arms of the four-wire line, but also to achieve a reflection-free termination of the two-wire line which a hybrid circuit of this kind, in a concrete construction which approximately achieves the desired resistance values, would not only appear relatively expensive, but also relatively critical in view of parasitic impedances which impede maintaining decoupling (bridge compensation) and freedom from reflection (matching). Outside a relatively narrow band range, this is even rendered impossible. Also, such a design would appear relatively critical with respect to overloading of the active elements of an impedance converter (in order to achieve a resistance value approaching zero) by interference signals occurring outside of the useful signal band, e.g. in the 50 Hz range.